000 WTNT41 KNHC 180837 TCDAT1 HURRICANE IGOR DISCUSSION NUMBER 41 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL112010 500 AM AST SAT SEP 18 2010 DATA FROM THE HURRICANE HUNTER AIRCRAFT INDICATE THAT WHILE THE CENTRAL PRESSURE OF IGOR HAD FALLEN TO 939 MB...THE WINDS AT FLIGHT LEVEL WERE NOT EFFICIENTLY MIXING DOWN TO THE SURFACE. THE INITIAL INTENSITY IS HELD AT 95 KT BASED ON A BLEND OF THE 115 KT FLIGHT- LEVEL WIND MAXIMUM AND A MAXIMUM WIND OF 77 KT FROM THE SFMR. THE OUTFLOW IS WELL ESTABLISHED...PARTICULARLY TO THE NORTH OF THE CENTER...AND THE ENVIRONMENTAL CONDITIONS APPEAR FAVORABLE FOR A LITTLE MORE STRENGTHENING OVER THE NEXT DAY OR SO BEFORE THE SHEAR INCREASES. THE OFFICIAL FORECAST SHOWS A PEAK AT 105 KT IN 24 HOURS AND THEN A SLOW WEAKENING THROUGH 48 HOURS. REGARDLESS OF THE DETAILS OF THE INTENSITY FORECAST...IGOR WILL BE A LARGE AND DANGEROUS HURRICANE AS IT APPROACHES BERMUDA ON SUNDAY. WEAKENING IS EXPECTED TO CONTINUE BEYOND 48 HOURS...AS IGOR MOVES OVER COOLER WATERS AND THE SHEAR INCREASES FURTHER. ONCE IGOR COMPLETES EXTRATROPICAL TRANSITION...IT WILL BE A LARGE AND POWERFUL CYCLONE OVER THE NORTH ATLANTIC. AFTER A SHORT TERM WOBBLE TOWARD THE WEST-NORTHWEST...THE LONG-TERM INITIAL MOTION ESTIMATE IS 310/11. IGOR IS SITUATED SOUTH OF A MID-LEVEL RIDGE EXTENDING ALONG 35N...AND THIS RIDGE IS FORECAST BY ALL OF THE MODELS TO WEAKEN AS A SHORTWAVE TROUGH PROGRESSES EAST OFF THE U.S. EAST COAST IN THE NEXT DAY OR SO. IGOR WILL TURN NORTHWARD INTO THE WEAKNESS ALONG 65W ON SUNDAY AND PASS VERY NEAR BERMUDA BEFORE TURNING NORTHEASTWARD ON MONDAY AHEAD OF ANOTHER SHORTWAVE TROUGH MOVING OFF THE COAST OF NORTH AMERICA. THE GLOBAL MODELS SHOW THE RIDGE NORTH OF IGOR REMAINING A LITTLE STRONGER THAN PREVIOUSLY FORECAST...AND AS A RESULT MOST OF THE GUIDANCE HAS SHIFTED A LITTLE TO THE LEFT AND SLOWER THROUGH 48 HOURS. THE OFFICIAL FORECAST HAS BEEN ADJUSTED IN THIS DIRECTION...BUT REMAINS TO THE RIGHT OF MOST OF THE GUIDANCE AND A LITTLE FASTER THAN THE CONSENSUS. BEYOND 48 HOURS...THE NEW NHC FORECAST IS LARGELY AN UPDATE OF THE PREVIOUS PACKAGE...AND SHOWS IGOR ACCELERATING TOWARD THE NORTHEAST AS IT BECOMES EXTRATROPICAL. IGOR HAS A VERY LARGE WIND FIELD...WITH A RADIUS OF MAXIMUM WINDS OF AROUND 45 NAUTICAL MILES AND HURRICANE FORCE WINDS EXTENDING OUT UP TO 90 NM FROM THE CENTER. CONDITIONS WILL BEGIN TO DETERIORATE ON BERMUDA BY TONIGHT...WELL IN ADVANCE OF THE CENTER. FORECAST POSITIONS AND MAX WINDS INITIAL 18/0900Z 25.1N 62.8W 95 KT 12HR VT 18/1800Z 26.2N 64.1W 100 KT 24HR VT 19/0600Z 27.8N 65.1W 105 KT 36HR VT 19/1800Z 29.9N 65.3W 100 KT 48HR VT 20/0600Z 32.2N 64.7W 95 KT 72HR VT 21/0600Z 38.5N 58.5W 80 KT 96HR VT 22/0600Z 47.5N 46.5W 65 KT...POST-TROP/EXTRATROP 120HR VT 23/0600Z 51.5N 39.5W 55 KT...POST-TROP/EXTRATROP $$ FORECASTER BRENNAN